Market pressures are causing gas suppliers to increase their purity standards in several electronic grade gases. The industry is demanding purer gases and verification of contaminant levels requiring improvements in methods for purifying and analyzing the gases supplied.
Conventional molecular sieves do not have sufficient gas capacity for weakly interacting adsorbates, such as nitrogen, to have practical utility. It is well known that due to their stronger interactions with zeolites, polar or polarizable adsorbates are selectively adsorbed from a mixture of less polarizable species. Adsorbates such as CO.sub.2, water, and the like are readily adsorbed in a variety of zeolites at low partial pressures even at ambient temperatures due to their large heats of adsorption. In theory, any adsorbent having a type I isotherm (which includes all zeolites) will have some capacity for a given adsorbate in the Henry Law region (i.e. at low partial pressures). However, only chabazite in the appropriate ion form and thorougly dehydrated has sufficient capacities for nitrogen and the like to have improved utility as a purifier. In addition, gases having significant induced or permanent dipoles such as CO or CO.sub.2 should have sufficiently high adsorption properties to allow removal from other bulk gases possessing lower affinities using conventional zeolites. However, the polyvalent chabazites should also be superior in these applications since they have greater capacity for a given adsorbate at low partial pressures.
In practice, the improved capacity results in either: (1) longer times between regenerations or replacements, or (2) the use of smaller beds in cyclic processes. The discovery of the intrinsic properties of polyvalent chabazites expands the range of adsorbates which can be removed from a bulk gas using standard adsorption processes.
The calcium form of mordenite is the only other zeolite known to be useful for removing N.sub.2 from argon. Compared to the mordenites, the chabazites of this invention are significantly better. Savage, in U.S. Pat. No. 3,597,169, teaches the use of molecular sieves to selectively adsorb liquid methane from liquid oxygen and claims that a wide variety of zeolites are useful, including chabazite. However, the specific examples and claims only cover certain ion forms of X zeolites. Unlike this process, the selective adsorption of weakly interacting adsorbates such as N.sub.2 at temperatures well above their liquefaction point is very dependent on the adsorption properties of a particular zeolite. In the applications of this invention, chabazite is in a class by itself. This is an unexpected result; among the more well-studied zeolites, chabazites seem to be unique in the magnitude of their capacity and selectively for weakly interacting adsorbates.
Titanium sponges held at 800.degree. C. are also used to remove nitrogen from gases. These adsorbents are irreversible and in addition these units only operate at very high temperatures and require the use of more expensive materials of special construction.